This invention relates to electronic positioning devices, and more particularly to an apparatus and method which provides for improved accuracy in the operation of position servo systems.
A position servo functions to position a device, hereafter referred to as a payload, to a selected location, hereafter referred to as a destination location, by moving the payload by a motion device, hereafter referred to as a prime mover. An associated control device operates to compare a current location of the payload with the destination location, and produce a control signal in accordance with a difference therebetween. The control signal is coupled to the prime mover to effect movement of the payload to the desired position.
As the control device orchestrates the overall operation of a position servo system, the accuracy with which the payload may be positioned is limited by the accuracy with which the control signal is produced. This in turn is limited by the accuracy with which position information can be determined. Position information is necessarily determined with respect to a selected reference point and typically relates to a difference between the desired location and a current location.
Position information is generally available to the control device on a continuous basis. It is to be understood in this regard that by use of the term continuous herein, it is understood that in analog systems, position information is available on a continuous basis. In a similar manner, in digital or other sampled systems where position information is available on a continual basis, i.e., at regular intervals, with interruptions between the intervals, the frequency at which position information is available is sufficiently high so that the effect is the same as if the position information were available on a continuous basis. Such position information is typically produced in an indirect manner, i.e., other than by position information from reference positions. By way of illustration, position information may be produced by coupling a position responsive transducer to the prime mover. The payload is then positioned at the reference position, and the signal from the position responsive transducer noted. As the payload is thereafter moved about positions removed from the reference position, the signal from the position responsive transducer is continuously monitored. Position information for the payload is thereby indirectly determined, and is available on a continuous basis, with reference position information available only at discrete position and/or only at discrete time intervals.
When position information is indirectly determined, it is necessary to periodically check the accuracy of such information. This is typically accomplished by moving the payload to a reference position and comparing the indirectly produced position information with the reference position information. The difference between the two represents the amount by which the indirectly produced position information is in error. As the payload is thereafter moved about positions removed from the reference position, the indirectly produced position information may be corrected by the amount of the error determined. Consequently compensation may be made for errors detected.
While the foregoing provides a method to monitor the accuracy of indirectly produced position information, there have been problems in the past with such an approach. In particular, there is generally a limit with respect to the amount by which indirectly produced position information may be in error and still be reliably corrected. Broadly speaking, if the error exceeds a maximum amount, it is not possible to reliably correct for the error.
While position servo systems are employed in a wide variety of applications, the foregoing discussed principles and shortcomings will be illustrated with respect to a hard disk storage system.
Broadly speaking, a hard disk storage system includes one or a plurality of disks which are rotated about an axis, which disks provide for the storage of information thereon in concentric rings or bands about varying radii across the surface thereof. Information is placed on and extracted from the surface of such disks by one or a plurality of magnetic transducers which are moved across the surface of the disk in response to control information. With respect to the foregoing terminology, the magnetic transducer would correspond to the payload, and the positioning apparatus which operates to move the payload across the surface of the disk would correspond to the prime mover. The destination loaction would correspond to selected tracks on the surface of the disks, and is provided to the system by external apparatus associated therewith. The location of the tracks across the surface of the hard disk respresent reference positions, with indirect position information being continuously produced for the magnetic transducers during movement between the tracks. Indirect position information of the magnetic transducer with respect to the surface of the hard disk may be provided in a number of different ways. In one approach, a disk surface and associated magnetic transducer are dedicated to position determination tasks. In particular, information is placed on concentric bands or tracks on the dedicated surface in such a fashion that continuous position information may be indirectly determined by the associated magnetic transducer. Signal multiplexing techniques may be further employed to facilitate the placement of several signals on a single track, thereby providing for improved accuracy in position determination.
In an alternate approach a position responsive transducer is employed. While a position responsive transducer may be implemented in any of a wide variety of ways, optical gratings, a light source and a light sensor are frequently employed. In such a system, one optical grating is affixed to a stationary member of the system, and a second coupled to the prime mover in such a fashion that motion of the prime mover produces relative motion between the two gratings. Light passing between the two gratings is monitored by the light sensor, and used to produce either a single or a plurality of signals useful for the indirect determination of position information of the payload, i.e., the magnetic transducer. Such optical gratings may operate in either a rectilinear or rotary fashion. In practice, a selected value of the signal from the light sensor corresponds to correct positioning of the magnetic transducer over a track. By comparing the signal from the light sensor with the corresponding selected value for the signal when the magnetic transducer is correctly positioned over a track, the accuracy of the position responsive transducer signal may be verified. In response to a difference between the signal from the light sensor and the corresponding selected value when the payload is correctly positioned at a reference position, a correction factor may be determined. Thereafter, position information for the transducer may be corrected in accordance with the amount of error so determined. In practice, however, the signal from the position responsive transducer is frequently linear only within a selected range. If the amount of detected error is within such range, the foregoing described correction technique may be employed. If, however, the error exceeds the linear range of the signal from the position responsive transducer, the required amount of correction may not be reliably determined.
By way of illustration, the signal produced by a position responsive transducer may be sinusoidal in nature, with zero crossings thereon corresponding to locations of tracks on the surface of the disc. Consequently, as the magnetic transducer is moved across the surface of the disk, the value of the sine wave will be zero when the magnetic transducer is directly positioned over each track. A number of practical considerations, however, operate to limit the reliability of this approach. In particular, as a result of environmental effects, including temperature, the disk will undergo thermal expansion and contraction, resulting in the relative displacement of the tracks with respect to position information from the position responsive transducer. As a consequence thereof, zero values of the sine wave will no longer correctly correspond to track locations on the surface of the disk. In particular, a non-zero value of the sine wave will correspond to correct track locations. In such a situation, the amount by which the signal from the position responsive transducer differs from the expected zero value of the sine wave represents a necessary correction factor. The correction factor is typically combined with position information from the position responsive transducer to determine correct position information. It is observed, however, that such a technique is limited in its application to situations wherein an approximate linear relationship exists between the signal produced by the position responsive transducer and the amount of the error. In the case of a sine wave, this is generally less than 45 degrees. When the amount of the error exceeds 45 degrees, the non-linearity present in the sine wave operates to prevent an accurate determination of the amount of error. The accuracy with which position of the magnetic transducer may be determined consequently quickly decreases.
There is consequently a need to provide for an improved technique for accurately determining position information over a wide range of environmental variations, and particularly with respect to hard disk drives.